In the instances that I have met people in person that use a double twisted wire bit, it has been because they were trying to cover up a training issue instead of going back and working on it. I'm not saying that's *always* the case, but it has been my experience so.
My trainer and several other people that I know do use a single twisted wire bit to fine tune a horse, but do not use it as their main bit. As they are very quiet and light in the hands and know what they are doing, I don't see a problem with it. I, however, would not feel comfortable using one myself.
Proud owner of ~Mana: 6yo Arabian gelding~Pearl 13yo Arabian~Danzer 14yo Arabian mare~ Tiny mini filly